Reticuloendotheliosis virus (REV) is a C-type RNA tumor virus which induces a rapidly lethal disease in chickens. REV is unrelated to the avian leucosissarcoma virus group, but represents the prototype of a new group of C-type RNA viruses. REV when propagated in chicken embryo fibroblast cultures, loses its oncogenic ability. Transformed bone marrow cultures have been isolated which chronically yield oncogenic virus. We propose to examine the basis of the attenuation of REV in fibroblast cultures, and to determine if REV RNA or REV transformed cells contain sequences specific for transformation. We will also continue to analyze the antigenic composition of REV looking for intraspecies specific determinants which cross react with other avian and murine tumor viruses. Antisera will also be prepared to the REV glycoproteins, and used to determine the extent to which REV is related to the nononcogenic members of this complex. In an effort to explain the extreme virulence of REV, the humoral and cellular immune response will be studied. We will also define whether chickens are tolerant to the REV group-specific antigen. With the antisera prepared against purified REV polypeptides, viral protein synthesis will be monitored, employing low ionic strength and cell fractionation procedures. BIBLIOGRAPHIC REFERENCES: Bishop, J.M., R.L. Maldonado, R.F. Garry, P.T. Allen, H.R. Bose, and M.R.F. Waite. 1976. The effect of medium of lowered NaCl concentration on virus release and protein synthesis in cells infected with reticuloendotheliosis virus. J. Virol. 17 (Feb). Maldonado, R.L. and H.R. Bose. 1976. Group-specific antigen among the members of the reticuloendotheliosis virus complex. J. Virol. 17 (March issue).